A Group-III nitride semiconductor light emitting device has been utilized as a light emitting device for a LED or LD, because this light emitting device has a direct transition-type band gap that extends over a range from visible light to ultraviolet light, thereby providing excellent light-emitting efficiency.
Furthermore, when used in an electronic device, the Group-III nitride semiconductor provides an electronic device having excellent properties, compared to a semiconductor using a conventional Group-III-V compound semiconductor.
Conventionally, in order to obtain a single crystal Group III nitride semiconductor wafer, a method has generally been used to grow the single crystal Group III semiconductor on a single crystal substrate, which is a heterogeneous material from the single crystal semiconductor. A large lattice mismatch is generated between a heterogeneous substrate and a Group-III nitride semiconductor crystal when it is epitaxially grown on the substrate. For example, when gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, the lattice mismatch therebetween reaches to 16%. When gallium nitride is grown on a SiC substrate, the lattice mismatch therebetween is as large as 6%.
In general, if such a big lattice mismatch is there, it is difficult to epitaxially grow a crystal directly on a substrate, or even if it is grown, a crystal having a favorable crystallinity, that is, a single crystal structure can be hardly obtained, and this raises a problem.
In order to solve the problem, a method has been proposed, wherein, when a Group-III nitride semiconductor crystal is epitaxially grown on a sapphire substrate or a SiC single crystal substrate by a metal-organic chemical vapor deposition (MOCVD) method, a so-called “low-temperature buffer layer” made of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) is first stacked on the substrate, and then, a Group-III nitride semiconductor crystal is epitaxially grown thereon at high temperature. This method has generally been adopted (for example, see Patent document 1 or 2).
Furthermore, the other method for forming the above-described buffer layer has been proposed by applying a method other than the MOCVD method. For example, a method is proposed where a buffer layer is formed by the sputtering method on a substrate including sapphire, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, a Group-III nitride-based compound semiconductor single crystals, or the like (for example, see Patent Document 3 or 4).
The methods disclosed in Patent Documents 1 to 4 cannot provide a Group-III nitride semiconductor having a sufficient crystallinity and this also raises a problem.
Furthermore, the other method has also been proposed, wherein, on a buffer layer formed by a high-frequency sputtering method, a crystal having the same composition as the buffer layer is grown by MOCVD (for example, Patent Document 5). However, the method in Patent Document 5 has a problem in that a favorable crystal structure cannot be stably formed on the substrate.
Still other method has also been proposed, wherein, an initial voltage of a sputtering apparatus is set to 110% or less of the sputtering voltage when a buffer layer is formed on the substrate by the sputtering method (for example, Document 6). The method described in Document 6 aims to form a buffer layer by the sputtering method, not using the MOCVD method which uses expensive raw materials.    Patent Document 1: Japanese Granted Patent No. 3026087    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H4-297023    Patent Document 3: Japanese Granted Patent No. 3440873    Patent Document 4: Japanese Granted Patent No. 3700492    Patent Document 5: Japanese Examined Patent Application, Second Publication No. H5-86646    Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2001-308010